Manufacturers of iron grids, aluminum windows, aluminum doors and products made of other materials at present typically use the known circular sawing machine as shown in FIG. 1A to cut the material, such as, plate, piping, etc. at the beginning of the work. In such processes, two types of saw blade are used, one is an emery wheel and the other is made of high speed steel. Both of the saw blades are replaceable in general, but the revolving speed of the motor driving the saw blade is much higher than is accustomed for the purpose of producing sufficient torque to counter-act the corresponding friction forces incurred in the course of cutting, reaching as high as 20,000 RPM. In such circumstances, when using an emery wheel as a saw blade, the higher cutting speed allows faster cutting of the material. But, since the emery wheel is a consumptive tool, particles of the emery wheel would fall off frequently, allowing the wheel to present a sharp cutting surface. The influence of the speed of the circular sawing machine using the emery wheel as saw blade isn't quite obvious, but problems would incur on replacing it with the saw blade of high speed steel material due to the cutting angle .theta. provided by the outer shape of the saw teeth at the peripheral edge of the steel saw blade. The cutting speed must be regulated relative to the hardness of the material to be machined, otherwise not only the circular sawing machine would fail to attain its most economical efficiency, but the fast cutting speed generates excessive shock force and temperature. As a result, damage to the saw teeth of the saw blade occurs.
Since the revolving speed of the saw blade of the circular sawing machine being directly driven by a main shaft of a motor is, in general, fixed and unchangeable, the saw teeth of the high speed steel saw blade are usually damaged, resulting in the shortening of the useful life of the blade, by fast speed of cutting of a working piece of harder material. The inventor has determined that the known devices have the following shortcomings (please refer to FIG. 1):
1. The conventional circular sawing machines are unable to change the revolving speed during cutting resulting in the above mentioned inconvenience in use.
2. On cutting the working piece by utilizing extremely high blade speed, the cutting surface of the working piece may be darkened, or even deformed and twisted due to friction heating, requiring the use of cutting oil. However, the use of cutting oil in a conventional sawing machine requires additional manual or mechanical means to supply the oil.
3. The jaw of the positioning vise 104 of a conventional circular sawing machine is unable to be replaced with one having an outer shape to match that of a special working piece, resulting in unstableness or failing to position the working piece appropriately. It is also unable to grip the working piece when cutting at an angle.